linux-kernel - Re: [RFC PATCH 0/2] remove SWAP_MAP

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Message-ID: <87o74cryhu.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date: Wed, 25 Sep 2024 15:19:57 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Nhat Pham <nphamcs@...il.com>
Cc: Yosry Ahmed <yosryahmed@...gle.com>,  Baolin Wang
 <baolin.wang@...ux.alibaba.com>,  akpm@...ux-foundation.org,
  hannes@...xchg.org,  hughd@...gle.com,  shakeel.butt@...ux.dev,
  ryan.roberts@....com,  chrisl@...nel.org,  david@...hat.com,
  kasong@...cent.com,  willy@...radead.org,  viro@...iv.linux.org.uk,
  baohua@...nel.org,  chengming.zhou@...ux.dev,  linux-mm@...ck.org,
  kernel-team@...a.com,  linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/2] remove SWAP_MAP_SHMEM

Nhat Pham <nphamcs@...il.com> writes:

[snip]

>
> My understanding now is that there are two for loops. One for loop
> that checks the entry's states, and one for loop that does the actual
> incrementing work (or state modification).
>
> We can check in the first for loop, if it is safe to proceed:
>
> if (!count && !has_cache) {
>     err = -ENOENT;
> } else if (usage == SWAP_HAS_CACHE) {
> if (has_cache)
>     err = -EEXIST;
> } else if ((count & ~COUNT_CONTINUED) > SWAP_MAP_MAX) {
>     err = -EINVAL;
> } else if (usage == 1 && nr > 1 && (count & ~COUNT_CONTINUED) >=
> SWAP_MAP_MAX)) {
>     /* the batched variants currently do not support rollback */
>     err = -ENOMEM;
> }
>
> At this point, IIUC, we have not done any incrementing, so no rollback
> needed? :)

I think that it's better to add a VM_WARN_ONCE() here.  If someone
enabled 'nr > 1' for __swap_duplicate(), the issue will be more
explicit.

[snip]

--
Best Regards,
Huang, Ying